1. Field of the Invention
The present invention relates to silicon nitride sintered products and processes for their production. More particularly, the present invention relates to silicon nitride sintered products which have excellent oxidation resistance and high strength at high temperatures and which are useful as materials for structural parts of various machines, instruments and equipments including automobiles, mechanical apparatus, chemical apparatus and aerospace equipments, and processes for producing such silicon nitride sintered products.
2. Discussion of Background
Sintered products containing silicon nitride as the main component i.e. silicon nitride sintered products are chemically stable at normal or high temperatures and have high mechanical strength, and they are expected to be used for sliding parts such as bearings or engine parts such as turbocharger rotors.
Heretofore, in order to obtain a silicon nitride sintered product having high strength, an oxide was added as a sintering aid to a silicon nitride powder, followed by firing at a temperature of at least 1,600xc2x0 C. to carry out liquid phase sintering and densification. Magnesium oxide, aluminum oxide and oxides of rare earth elements are known as oxides effective as sintering aids. Among them, magnetism oxide, aluminum oxide and yttrium oxide are commonly used alone or in the form of a mixture during the firing. Such a sintering aid reacts with silicon oxide as an oxide layer on the surface of the material at a high temperature, to form a liquid phase. Sintering proceeds as silicon nitride will diffuse in the liquid phase thus formed. Upon cooling after the sintering, the majority of the liquid phase will remain at the grain boundaries in the form of a glass phase, although a part of the liquid phase will be crystallized as an oxide or an oxynitride. Accordingly, a silicon nitride sintered product is usually composed of silicon nitride grains and a glass phase as the grain boundary phase.
However, when such a sintered product is used in a high temperature environment of at least 1,000xc2x0 C., there has been a problem that the glass phase at the grain boundaries softens, whereupon the strength rapidly decreases. The degree of the decrease of strength at a high temperature depends very much on the chemical composition of the grain boundary phase, as the softening temperature of glass is proportional to the melting point of the metal-Sixe2x80x94O system in the grain boundary phase. Accordingly, the high temperature strength or creep resistance will be high when a mixture of aluminum oxide and yttrium oxide is incorporated rather than when magnesium oxide is incorporated as a sintering aid.
Recently, a study has been made on a system wherein a mixture of a rare earth oxide and silicon oxide, is used as a sintering aid. For example, J. Am. Ceram. Soc. No. 75, p. 2050 (1992) reported on a silicon nitride sintered product having high melting point Y2Si2O7 precipitated at the grain boundaries by adding a sintering aid of a yttrium oxide-silicon oxide type. In this silicon nitride sintered product, a nitrogen-containing apatite (N phase, Y10Si7O22N4) or K phase (YSiO2N) as shown in the phase diagram in FIG. 4, or a glass phase of a composition close thereto, constitutes a second grain boundary phase following Y2Si2O7. The softening temperature of the N phase or the K phase is not higher than 1,500xc2x0 C., whereby the high temperature strength or the creep resistance of the silicon nitride sintered product having such a phase or a glass phase similar thereto as the grain boundary phase was not fully satisfactory. Further, also in a phase diagram of a Si3N4xe2x80x94Y2Si2O2xe2x80x94Si2N2O ternary system, compositions in and around the triangle having the respective components at its apexes were studied, but the high temperature strengths were not adequate.
Further, with respect to a Si3N4xe2x80x94SiO2-RE2O3 (RE: rare earth element) ternary system, JP-A-4-15466 discloses that the J phase (RE4Si2O7N2), the N phase and the K phase as shown in the phase diagram in FIG. 4, were precipitated at the grain boundaries; JP-A-4-243972 discloses that the J phase and a rare earth nitride were precipitated at the grain boundaries; and JP-A-4-292465 discloses that the S phase (RE2SiO5) was precipitated at the grain boundaries. Further, JP-A-8-48565 discloses that the J phase, or two phases i.e. the J phase and the S phase, as shown in the phase diagram in FIG. 5, were precipitated as the grain boundary phase.
However, in each case, it was necessary to add a large amount of a rare earth oxide to control the composition, and the amount of the grain boundary phase increased, whereby there was a new problem that the product was susceptible to oxidation at a high temperature, thus leading to deterioration of the creep resistance and the oxidation resistance. Further, a special heat treatment was required for the crystallization. These publications disclose nothing about the differences in the effect of incorporation among various rare earth elements.
Under these circumstances, it is an object of the present invention to provide a silicon nitride sintered product having excellent oxidation resistance and high strength at a high temperature, at the same time, and a process for producing it.
Another object of the present invention is to provide a material excellent in creep resistance by efficiently crystallizing a high melting point sintering assistant at the grain boundaries by means of a common sintering method requiring no special heat treatment, by studying a precise composition relating to the type and the amount of the rare earth element, thereby to solve the above-mentioned problems of the prior art.
A sintering aid is required to form a liquid phase at a temperature not higher than the sintering temperature in order to facilitate the liquid phase sintering and to remain as a crystal phase having a high melting point after the sintering. These two points are essential to obtain a sintered product having high heat resistance, and, in many cases, this is the reason why a rare earth oxide is used as a sintering aid. However, as mentioned above, the phase diagram of a Si3N4xe2x80x94SiO2-RE2O3 system is complex, and it has been difficult to produce a sintered product having a grain boundary phase composed solely of the desired high melting point phase.
Under the circumstances, the present inventors have paid an attention to the differences in the phase diagrams and the sinterability among various rare earth elements and have succeeded in letting the J phase (Lu4Si2O7N2) precipitate at the grain boundaries even by an addition of a small amount of Lu oxide, by selecting lutetium (Lu) as the rare earth element and by removing the oxygen impurity in a silicon nitride powder as the starting material, and they have found it possible to obtain a silicon nitride sintered product which has not only high strength but also excellent oxidation resistance. Likewise, they have succeeded in letting Lu2SiO5 phase precipitate efficiently even by a common sintering method requiring no special heat treatment, by selecting lutetium as the rare earth element and by controlling the composition precisely, and they have found it possible to thereby obtain a sintered product excellent in creep resistance.
Thus, in the first aspect, the present invention provides a silicon nitride sintered product comprising silicon nitride grains and a grain boundary phase, wherein the grain boundary phase consists essentially of a single phase of a Lu4Si2O7N2 crystal phase, and the composition of the silicon nitride sintered product is a composition in or around a triangle ABC having point A: Si3N4, point B: 28 mol % SiO2-72 mol % Lu2O3 and point C: 16 mol % SiO2-84 mol % Lu2O3, as three apexes, in a ternary system phase diagram of a Si3N4xe2x80x94SiO2xe2x80x94Lu2O3 system.
In the second aspect, the present invention also provides a silicon nitride sintered product comprising silicon nitride grains and a grain boundary phase of an oxynitride, wherein the composition of the sintered product is a composition in a triangle having point A: Si3N4, point B: 40 mol % SiO2-60 mol % Lu2O3 and point C: 60 mol % SiO2-40 mol % Lu2O3, as three apexes, in a ternary system phase diagram of a Si3N4xe2x80x94SiO2xe2x80x94Lu2O3 system.